1. Field of the Invention
The present invention relates to a method of annealing a semiconductor film by using a laser beam (hereinafter referred to as laser annealing). The invention also relates to a semiconductor device fabricating method which includes the laser annealing method as one step. Incidentally, the term “semiconductor device” used herein generally denotes devices which can function by using semiconductor characteristics, and encompasses electrooptical devices such as liquid crystal display devices and luminescent devices as well as electronic equipment including the electrooptical devices as constituent parts.
2. Background Art
In recent years, a wide range of researches have been made as to the art of applying laser annealing to a semiconductor film formed on an insulating substrate such as a glass substrate to crystallize the semiconductor film or to improve the crystallinity thereof. Silicon is widely used for such a semiconductor film. In the present specification, means for crystallizing a semiconductor film by a laser beam to obtain a crystalline semiconductor film is referred to as laser crystallization.
As compared with synthetic quartz glass substrates which have heretofore widely been used, glass substrates have the advantages of being inexpensive and rich in workability and of facilitating fabrication of large-area substrates. This is the reason why a wide range of researches have been made. The reason why lasers are preferentially used for crystallization is that the melting points of glass substrates are low. Lasers can give high energy to semiconductor films without increasing the temperatures of substrates to a great extent. In addition, lasers are remarkably high in throughput compared to heating means using electric heating furnaces.
A crystalline semiconductor is made of multiple crystal grains, and is also called a polycrystalline semiconductor film. Since a crystalline semiconductor film formed by the application of laser annealing has high mobility, the crystalline silicon film is used to form thin film transistors (TFTs). The thin film transistors are widely used in a monolithic type of liquid crystal electrooptical device in which TFTs for pixel driving and TFTs for driver circuits are fabricated on one glass substrate.
A method of effecting laser annealing by forming a high power pulsed laser beam such as an excimer laser beam, by an optical system, into a laser beam which becomes a spot of several cm square or a linear shape of length 10 cm or more at an irradiation plane, and scanning the laser beam (or relatively moving a position irradiated with the laser beam with respect to an irradiation plane) has preferentially been used because the method is high in productivity and superior in industrial terms.
Particularly when a linear laser beam is used, high productivity can be realized because the entire irradiation plane can be irradiated with the linear laser beam by scanning in only directions perpendicular to the lengthwise direction of the linear laser beam, unlike the case where a spot-shaped laser beam is used which needs to be scanned in forward, rearward, rightward and leftward directions. The reason why the linear laser beam is scanned in the lengthwise direction is that the lengthwise direction is the direction of the most efficient scanning. Because of this high productivity, in the laser annealing method, the use of a linear laser beam into which a pulse oscillation excimer laser beam is formed by an appropriate optical system is presently becoming one of leading manufacturing techniques for semiconductor devices which are represented by liquid crystal devices using TFTs.
Although there are various kinds of lasers, it is general practice to use laser crystallization due to a laser beam which uses a pulse oscillation type of excimer laser as its light source (hereinafter referred to as an excimer laser beam). The excimer laser has high power and hence the advantage of enabling irradiation repeated at high frequencies, and further has the advantage of exhibiting a high absorption coefficient against silicon film.
To form the excimer laser beam, KrF (of wavelength 248 nm) and XeCl (of wavelength 308 nm) are used as exciting gases. However, gases such as Kr (krypton) and Xe (xenon) are very expensive and encounter the problem that as the frequency of gas replacement becomes higher, a greater increase in manufacturing cost is incurred.
Attachments such as a laser tube for effecting laser oscillation and a gas purifier for removing unnecessary compounds generated in an oscillation process need to be replaced every two or three years. Many of these attachments are expensive, resulting in a similar problem of an increase in manufacturing cost.
As described above, a laser irradiation apparatus using an excimer laser beam surely has high performance, but needs extremely complicated maintenance and also has the disadvantage that if the laser irradiation apparatus is used as a production-purpose laser irradiation apparatus, its running costs (which mean costs occurring during operation) become too high.
There is a method which uses a solid-state laser (a laser which outputs a laser beam by means of a crystal rod formed as a resonance cavity), to realize a laser irradiation apparatus which is low in running cost compared to excimer lasers as well as a laser annealing method using such a laser irradiation apparatus.
A semiconductor film was irradiated by using a YAG laser which was one of representative solid-state lasers. The output from the YAG laser was modulated into the second harmonic by a non-linear optical element, and the resulting laser beam (of wavelength 532 nm) was formed into a linear laser beam which became a linear shape at an irradiation plane. The semiconductor film was an amorphous silicon film of thickness 55 nm which was formed on a #1737 glass substrate made by Corning Incorporated, by a plasma CVD method. However, a concentric-circle pattern such as that shown in FIG. 2 was formed on the crystalline silicon film obtained by effecting laser annealing on the amorphous silicon film. This pattern indicates that the in-plane properties of the crystalline silicon film is non-uniform. Accordingly, if a TFT is fabricated from a crystalline silicon film on which a concentric-circle pattern is formed, the electrical characteristics of the TFT is adversely affected. Incidentally, in the present specification, a pattern such as that shown in FIG. 2 is called a concentric-circle pattern.